The present invention relates to a control device of a blower motor in an automobile air-conditioning system.
FIG. 1 shows a prior art blower motor speed control device for an automobile air-conditioning system.
In FIG. 1, reference numeral 1 shows a blower motor. In a power supply circuit of the blower motor 1, a resistor circuit comprises resistors 2, 3 and 4 for speed control. The resistors 2, 3 and 4 provide a variable resistance that can be selected by either an automatic speed changing means including speed changing relays 5, 6 and 7, switching transistors 8, 9 and 10 and comparators 11, 12 and 13, or by a manual speed changing switch 14. The automatic speed changing means and the manual speed changing switch 14 can be selected by switching means comprising a relay 15 and a changeover switch 16. When the changeover switch 16 is switched at a contact (A), a common contact of the manual speed changing switch 14 is grounded, and speed of the blower motor 1 is changed manually by the manual speed changing switch 14. When the changeover switch 16 is switched at a contact (B), the relay 15 is closed, and speed of the blower motor 1 is changed automatically by operation of the speed changing relays 5, 6 and 7. Reference voltages supplied to negative input terminals of the comparators 11, 12 and 13 in the automatic speed changing means are different from each other, their values being reduced in order of the comparators 11, 12 and 13. To positive input terminals of the comparators 11, 12 and 13, a thermal load signal is supplied. The thermal load signal, which is calculated and supplied based on a set room temperature, a detected room temperature, a detected ambient temperature and a detected quantity of solar radiation, represents a quantity of thermal load to be controlled to attain a set temperature in a room. The voltage level of the thermal load signal will rise in accordance with an increase of the thermal load to be controlled. The comparators 11, 12 and 13 supply Low level outputs when the voltage level of the thermal load signal is low, but as the thermal load signal increases, their outputs change to a High level in order of the comparators 11, 12 and 13. The transistors 8, 9 and 10 become conductive in response to the High level outputs of corresponding comparators 11, 12 and 13. The speed changing relays 5, 6 and 7 change from contacts (a) to (b) in response to conduction by corresponding transistors 8, 9 and 10. If the changeover switch 16 is switched to the contact (B), the speed of the blower motor 1 is changed automatically by the operation of the speed changing relays 5, 6 and 7.
In the prior art controlling device composed as above, the relays 5, 6 and 7 are not in their off state under the manual operation mode with the changeover switch 16 switched to the contact (A), therefore, the speed changing relays 5, 6 and 7 operate and generate their operating sound despite the use of the non-automatic speed changing control device. Consequently, the prior art controlling device not only give jarring sounds to a driver and passengers, but also give a misunderstanding to a driver and passengers as if the air conditioner was in the automatic speed changing node.